Staged Sodar Sounding

Abstract

A monostatic sodar system (10) for atmospheric sounding includes a processor and display unit (12) that generates a set of acoustic chirps for transmission by transmitter 20. Discontinuities (22, 24 and 26) result in echoes (28, 30 and 32) being returned to a receiver (38) of system (10). Receiver (38) outputs extracted echo signals on line to the processor (12) for analysis. Detector (36) implements a Fourier domain matched-filter to extract echo signals from noise. By using a set of multiple chirps of increasing length with increasing intervals between them, substantially any feasible range can be accommodated using send-then-listen techniques with the benefit of high s / n performance.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]This invention relates to sodar methods and apparatus for atmospheric sounding using encoded pulses in the transmit-then-listen mode. The techniques of the invention have particular application to so-called ‘monostatic’ sodar configurations in which the receiver(s) is located ‘up-range’ near the transmitter and backscattered echoes are detected by the receiver. However, the methods of the invention are also applicable to so-called ‘bistatic’ sodar configurations where the receiver(s) is located down-range and forward-scattered echoes are detected.[0003]2. Description of Related Art[0004]Transmit-then-listen sodar sounding techniques, in which the transmission of a short high-intensity acoustic pulse is followed by a longer listening period in which echoes from the pulse are detected, are standard in the prior art and have been known for nearly 100 years. The basic problem with such techniques is that the pulse must be v...

AI Technical Summary

Benefits of technology

[0008]Though not essential, the coding of the successive pulses is preferably substantially the same and the coding method is preferably adapted for matched filter processing. In that case, such pulses can be called chirps within the meaning employed in our prior patent applications. In one preferred option, each pulse may be frequency modulated so that the frequency increases or decreases in a linear manner for the duration of the chirp. Preferably each pulse has the same starting frequency or tone and the same finishing frequency or tone. The rate at which the tone increases (or decreases) will then, of course, decline as the pulses get longer, but each pulse will have effectively the same bandwidth. This allows greater simplicity in the design of the receiver system because the bandwidth of each pulse is the same and the same matched filter, together with and much of the same DSP processes, can be used to extract echo data from the received signals following the end of the transmission of each pulse.

Problems solved by technology

The basic problem with such techniques is that the pulse must be very short—preferably a few milliseconds—so that the transmit pulse ends before the echoes from short-range anomalies are returned and detected, otherwise the energy of the transmitted pulse will swamp the receiver.

Two problems associated with the use of short pulses: they must have very high energy to achieve an acceptable signal-to-noise ratio [s / n], and they are difficult to encode in any effective way so that pulse-compression techniques are difficult to apply.

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